Ethylene oxide catalyst

ABSTRACT

The invention is directed to improvements in the production of ethylene oxide by the controlled, partial oxidation of ethylene in the presence of a silver supported catalyst. New results and advantages including elimination of the need for halogenated inhibitors in the ethylene oxide process are attributable to the use of a silver impregnated catalyst comprising inert porous support particles wherein at least 90 percent of the pore diameters fall within a limited range and the average pore diameter falls within a narrow range. The invention is also attended by supplemental benefits in the form of increased productivity and/or reduced operating temperature.

United States Patent De Maio May 23, 1972 [54] ETHYLENE OXIDE CATALYST3,258,433 6/1966 Lambert et al ..252/454 x 3,420,784 1/1969 Keith et al...252 454 [72] Denms De State Island 3,501,417 3/1970 De Maio ..252/454[73] Assignee: Halcon International, Inc.

Primary Examiner-C. F. Dees [221 Fllcdl P 1969 ArtorneyWilliam c. Longand David Dick [21] Appl. No.: 817,169

Related US. Application Data [63] Continuation-impart of Ser. No.723,915, Apr. 24,

1968, abandoned.

[52] US. Cl ..252/454, 252/476, 260/348.5

[5 l] Int. Cl. ..B01j 11/40 [58] Field of Search ..252/454, 455, 476;260/3485 F [56] References Cited UNITED STATES PATENTS 2,799,687 7/1957Gould et a1. ..260/348.5 3,207,700 9/1965 Saffer ..252/454 X [57]ABSTRACT The invention is directed to improvements in the production ofethylene oxide by the controlled, partial oxidation of ethylene in thepresence of a silver supported catalyst.

New results and advantages including elimination of the need forhalogenated inhibitors in the ethylene oxide process are attributable tothe use of a silver impregnated catalyst comprising inert porous supportparticles wherein at least 90 percent of the pore diameters fall withina limited range and the average pore diameter falls within a narrowrange.

The invention is also attended by supplemental benefits in the form ofincreased productivity and/or reduced operating temperature.

4 Claims, No Drawings ETHYLENE OXIDE CATALYST This application is acontinuation-in-part of application Ser. No. 723,915, filed Apr. 24,1968 which is now abandoned.

This invention relates to the improved production of ethylene oxide andparticularly to ethylene oxide catalysts and support material for suchcatalysts. More particularly, the invention concerns the use of inert,porous material having a limited range of pore diameters with a narrowaverage pore diameter range as support material for silver impregnatedcatalysts in the controlled, partial oxidation of ethylene to ethyleneoxide. The invention also concerns an improved ethylene oxide processwherein said oxidation is efiected in the presence of such catalysts andin the absence of halogenated inhibitors.

In general, commercial production of ethylene oxide is accomplished bythe vapor phase reaction of ethylene with molecular oxygen in thepresence of a silver catalyst. The silver is usually in finely dividedmetallic form distributed on or throughout an inert support or carriermaterial. The latter are indicated in the prior art as including suchmaterials as clay, bauxite, bentonite, kieselguhr, active carbon pumicestone, silica and fused alumina. The manner of application of the silverto the support material and its effect on ethylene oxide productivityhas been the subject of numerous investigations. In this regard, U.S.Pat. No. 3,172,893 discloses a method of application wherein silver iscoated onto the surfaces of support particles by spraying with a slurryof precipitated silver oxide. British Pat. No. 754,593 discloses anothermethod of silver application wherein silver is impregnated throughoutthe pores of the carrier by immersion of the latter in an aqueous silversolution, followed by drying and reduction to metallic silver.

Experience with the aforementioned silver catalysts has met with varyingdegrees of success, notwithstanding the close control of reactionconditions which the sensitive nature of the ethylene oxide processrequires. Investigations have shown that results often vary depending onthe method of silver deposition as well as the characteristics of thesupport material employed without a clear indication of the catalystmechanism. The apparent complexity of such mechanism is compounded bythe universal need for tempering the catalyst activity with ahalogenated inhibitor, such as ethylene dichloride. The latter generallycomprises a portion of the gaseous feed mixture to the reaction zoneand, while incremental relative to the quantities of ethylene and oxygenconsumed during production of ethylene oxide, nevertheless, is a costfactor.

The economic penalty attendant the employment of a halogenated inhibitorin the ethylene oxide process, however, is not measured by this costfactor alone. The metering of such inhibitor requires very carefulcontrol and regulatory apparatus together with the expense necessarilyincurred by expanded technical personnel requirements account for apenalty significantly in excess of the cost of the inhibitor materialalone. The presence of a halogenated inhibitor in the ethylene oxideprocess necessarily interferes with product purity requiring additionalsteps to effect removal or separation. Process control suffers to someextent since an additional variable is introduced to the system. Ofprime importance, however, is the fact that the presence of ahalogenated inhibitor necessitates the employment of mere expensivematerials of construction to withstand the corrosive nature of thematerial. Additional disadvantages any be attributed to the use ofhalogenated inhibitors including the necessity of taking neutralizationmeasures where high purity oxygen is employed to effect the partialoxidation of ethylene.

Accordingly, a principal object of the present invention is to provide asupport material for catalysts used in the controlled, partial oxidationof ethylene to ethylene oxide which avoids the necessity of employing ahalogenated inhibitor.

Another object of the invention is to provide a silver impregnatedcatalyst for use in the controlled oxidation of ethylene to ethyleneoxide which obviates the need of a halogenated inhibitor.

A further object of the invention is to provide a catalyst for thecontrolled oxidation of ethylene to ethylene oxide which comprises asilver impregnated porous support permitting improved productivityand/or lower operating temperature.

Still another object of the invention is to provide an improved ethyleneoxide process wherein controlled oxidation of ethylene to ethylene oxideis effected in the presence of the aforementioned inventive catalyst andin the absence of a halogenated inhibitor.

Other objects of the invention will in part be obvious in part appearhereinafter in the following detailed description and includedembodiments.

The present invention is concerned essentially with a unique supportmaterial for silver-containing catalysts for use in the production ofethylene oxide. The invention stems from the discovery that use of asupport material which is porous in nature and characterized by alimited range of pore diameters, the average pore diameter of whichfalls in a narrow range, can eliminate the heretofore universal need forhalogenated inhibitors to temper or otherwise control the activity ofthe silver-containing catalysts employed in the controlled, partialoxidation of ethylene to ethylene oxide. The invention contemplates thatethylene oxide catalysts prepared with the unique support material willbe of the impregnated" type, that is, the catalyst is formed by fillingthe pores of the support material with a silver solution from which thesilver solute is deposited. A discussion of an illustrative techniqueand procedure employed to prepare such a catalyst is provided in theaforementioned British Pat. No. 754,593.

The principles underlying the present invention are admittedly complexand the particular centers for catalyzing the different avenues ofethylene oxidation are not fully defined. Theoretically, some catalystcenters exhibit activity in the production of ethylene oxide fromethylene, while others exhibit activity in the formation of carbondioxide from ethylene. It may also be conjectured that catalyst centersexist which effect the further oxidation of ethylene oxide to acidaldehydes and carbon dioxide. The picture is further com plicated by thefact that realistic reaction rates and conversions attend hightemperatures while low temperatures promote the selectivity of ethyleneoxide. Normally, sufficiently high reaction temperatures are employed toprovide an economically feasible ethylene conversion while a halogenatedinhibitor, such as ethylene dichloride, is employed in the cycle gas toselectively poison those catalyst centers which exhibit undesirableactivity, e.g., exhibit activity in the formation of carbon dioxide.While applicants do not intend to be bound thereby, in principle, itwould appear that catalyst centers exhibiting undesirable activity areminimized by the more homogeneous distribution of silver obtained bydeposition thereof upon a porous support material wherein a substantialportion of the pores have diameters which fall within a limited rangeand wherein the average pore diameter falls within a narrow range.

For the purpose of the invention a support material may be regarded asporous when it has a minimum apparent porosity of about 30 percent. Itmay be noted that apparent porosity may be determined by the mercuryporosimeter method described by Drake and Ritter in The AnalyticalEdition of Industrial Engineering Chemistry, Vol. 17, page 787 (1945).

A substantial portion of the pores" may be regarded as embracing aminimum of about percent of the total pores of the support material.

The invention contemplates a limited range of pore diameters asincluding a maximum pore diameter range of about 30 microns. Porediameters may be determined by recourse to the BET. nitrogen absorptionmethod described by Brunauer, Emmett and Teller in The Journal of theAmerican Chemical Society," Vol. 60, page 309 (1938) and the Gurvichaequation described in The Journal of the Russian Physical ChemicalSociety, Vol. 47, page 805 1915 The invention also contemplates that theaverage pore diameter" will be of a size such that neither too low nowtoo high a diffusion rate is encountered in practice. All other factorsbeing equal, the former is attended by poor ethylene oxide selectivitywhile the latter is attended by poor catalyst activity. it has beendetermined that an average pore diameter range of 4 to microns incombination with the aforementioned catalyst support characteristicspermits a diffusion rate giving the optimum combination of ethyleneoxide selectivity and catalyst activity without the necessity ofemploying a halogenated inhibitor.

Practice of the ethylene oxide process using the inventive catalyst andin the absence of a halogenated inhibitor may be conducted with aconsiderable degree of flexibility. Operation of the process may proceedat significantly reduced reaction temperatures while maintaining currentproductivity levels. Such operation might be dictated where maximumservice life is a controlling factor. On the other hand, significantlyincreased productivity can be achieved without exceeding currentoperating temperatures.

In general, the inventive catalyst support may comprise any porous,inert material which does not detrimentally influence the catalyticreaction wherein it is employed. Accordingly, in the catalytic oxidationof ethylene to ethylene oxide, suitable support materials include alphaalumina, silicon carbide, fused aluminum oxide, mixtures of alumina andsilica, and the like. Desirably, the support material comprises 90percent or more by weight alpha alumina and 1 to 6 percent by weightsilica. A preferred support material comprises 90 percent or more byweight alpha alumina, l to 6 percent by weight silica and 0.1 to 0.4percent by weight baryta.

The form of the inventive support may be described as a pellet havinglinear and diametrical dimensions suitably within the range ofone-eighth to five-sixteenths inch and may take almost any solidgeometrical configuration. From the standpoint of simplicity, suchconfiguration is suitably cylindrical, spheroidal or spherical. It isrecognized that engineering factors, such as ability to pack uniformly,mechanical strength, pressure drop and stability, may influence thechoice of configuration and, accordingly, it is contemplated that morecomplex configurations, such as saddles and rings, may be dictated.

The catalyst supports are prepared as follows: The support material,e.g., high-purity aluminum oxide particles, preferably in the alphaalumina phase, are thoroughly mixed with temporary and permanentbinders. The temporary binders are thermally decomposable organiccompounds of moderate to high molecular weight (i.e., molecular weightsabove about 250) and, on decomposition, produce the pore structure ofthe support. The permanent binders are inorganic clay-type materialshaving fusion temperatures below that of the alumina and are responsiblefor imparting mechanical strength to the finished support. Silica andbaryta can also be added in quantity sufficient to obtain a finishedsupport of the desired composition. After thorough dry-mixing,sufficient water is added to the mass to form the mass into a paste-likesubstance. The catalyst support particles are then formed from the pasteby conventional means such as, for example, high pressure extrusion,granulation or other ceramic forming processes. The particles are thendried and are subsequently fired at an elevated temperature which is inthe range of 1,200 to 1,600 C.

In the firing step, the temporary binders are thermally decomposed tocarbon dioxide and water and are volatilized, leaving voids in thesupport mass. These voids are the genisis of the pore structure of thefinished support. Suitable temporary binders include such materials asthe celluloses and substituted celluloses, e.g., cellulose itself,methyl cellulose, ethyl cellulose, and carboxyethyl cellulose, stearatessuch as organic stearate esters, e.g. methyl or ethyl stearate, waxesand the like. As firing is continued, the temperature reaches the pointat which the permanent binder (inorganic clay such as the kaolins or theball clays) fuses. The catalyst support is then permitted to cool and,during cooling, the permanent binder sets and acts as a cement to bondthe catalyst support particles and thereby impart mechanical strength tothe support and ensure maintenance of the pore structure.

Catalyst supports of desired characteristics can be readily produced bythe foregoing procedure. Control of pore size, pore size distributionand porosity are readily effected by appropriate adjustment in knownmanner of the size of the starting alumina particles, and of theparticle size and concentration of the temporary and of the permanentbinders in the starting mix. The larger the starting alumina particlesize, the greater will be the porosity of the finished catalyst. Themore homogeneous in size are the alumina particles, the more uniformwill be the pore structure. Similarly, increasing the concentration ofthe temporary binder will also increase the overall porosity of thefinished catalyst support.

As aforementioned, the support material of the present invention isintended for use in the preparation of ethylene oxide catalysts of theimpregnated type rather than the coated type. lmpregnation may suitablybe accomplished by immersion of the support material in a silversolution, e.g., an aqueous solution of silver nitrate or silver lactate.Essentially, the solution with the support material immersed therein isheated, desirably to boiling, to effect degasification of the supportmaterials and to insure complete and uniform impregnation thereof. Theimpregnated support material may then be removed for the excesssolution, and carefully and slowly dried.

Deposited silver salt is then converted to a catalytically active form,e.g., by hydrogen reduction in a case such as silver nitrate or bythermal decomposition in a case such as silver lactate.

The amount of silver deposited upon the support material will becontrolled by the concentration of the silver salt in the impregnatingsolution and by the porosity of the catalyst support. The actual amountof silver on the support after reduction or thermal decomposition canvary from about 5 percent to about 30 percent by weight of the reducedor thermally decomposed impregnated catalyst, the minimum value beingdictated by insufficient activity while the maximum value represents thepoint where the catalyst begins to resemble a coated catalyst inphysical properties.

Promoters are often employed in ethylene oxide catalysts in combinationwith silver catalytic material for the primary purpose of extendingcatalyst service life while also influencing particular aspects of theethylene oxide reaction. In this regard, materials comprising barium,copper, mercury or tin are illustrative of promoters commonly employedin the catalystic oxidation of ethylene to produce ethylene oxide andcontemplated by the present invention. Such materials are generallyintroduced to the silver catalytic material-containing media employed toimpregnate the support and thus are generally co-deposited with thesilver catalytic material.

In practice, a water-soluble salt of the promoting metal is added to thesilver salt solution in such proportions that the concentration ofpromoter metal (based on the equivalent metal content of the oxide) isfrom about 0.1 percent to about 3.0 percent by weight of the silvermetal content of the reduced catalyst. Amounts of promoter metal lessthan about 0.1 percent of the silver generally exert a negligible effecton catalyst life while amounts higher than about 3 percent lead toreactions which are difficult to control. The preferred concentration ofpromoter metal is from 0.1 to 1.5 percent by weight of the silver metal.The water-soluble salt of the promoter metal is from 0.1 to 1.5 percentby weight of the silver metal. The water-soluble salt of the promotermetal, of course, should be consonant with the technique employed totreat the deposited silver salt. Thus, in the case of reduction, thesalt 5 should be reducible to the oxide at the catalyst reductiontemperatures employed. For this purpose, the nitrite and nitrate saltsare suitable. In the case of thermal decomposition, the salt should bethermally decomposable at the thermal decomposition temperaturesemployed. For this purpose, the lactate and oxalate salts are suitable.

The pellets were then impregnated with silver by immersion in asilver-containing solution prepared as follows: 1,000 parts by weight ofsilver oxide is added slowly with stirring to 1,224 parts by weight ofan 85 percent solution of lactic acid in water over a period of 30 to 45minutes, the mixture being continuously cooled to avoid temperatures inexcess of 95 C. and thereafter 100 parts by weight of a percent solutionof hydrogen peroxide in water is slowly added to produce a clear yellowsolution, whereupon 38 parts by weight of a 44.4 percent barium lactatesolution in water is added. The silver-containing solution is maintainedat a temperature of 90 to 95 C. during immmersion of the pellets, whichare permitted to soak for 5 to 15 minutes with occasional stirring. Thepellets are then separated from the solution by draining. After about 15minutes of draining, the pellets are dried for at least 10 hours at atemperature of 60 to 70 C in an air atmosphere. The dried pellets arethen heated gradually over a period of about 4 hours to a temperature ofabout 250 C. in an air atmosphere and maintained at such temperature foran additional 4 hours to effect activation.

Evaluation of the catalyst pellets, prepared as aforedescribed, wasconducted at primary reactor conditions in a pressure reactor system forthe controlled oxidation of ethylene to ethylene oxide. Feed gascomprising 5.0 percent ethylene, 6.0 percent oxygen, 0.1 percent ethane,balance essentially nitrogen, was passed at a flow rate of 53.5lbs/hr/tube and a pressure of 315 psia through the prepared catalyst ata bed height of 24 ft 5 in. The results in terms of ethylene oxideselectivity and productivity obtained at recited reactor temperaturesand halogenated inhibitor contents are set forth in Table II. It may benoted at this point that the term selectivity" is commonly employed toindicate the percentage of converted ethylene constituting ethyleneoxide, such value being of controlling importance in the operation of aprimary or main reactor. The term productivity" has a more generalconnotation but may be regarded simply as the difference between productoutput and product input across the reactor. In this regard, ethyleneoxide productivity is commonly measured indirectly by A i.e., thedifference between the ethylene oxide concentration in the reactoreffluent and inlet gases.

TABLE 11 A B C D Reactor temperature, C. 245 235 245 220 230 EthyleneDichlon'de, ppm 0.03 0.04 0 0 0 Selectivity AEO Analysis of the data ofTable 11 indicates quite clearly that catalysts prepared with support D,i.e., the inventive support, permit the operation of the ethylene oxideprocess at significantly lower reaction temperatures, e.g., 220 to 230C, without the necessity of employing a halogenated inhibitor.Comparison of inventive support D with support C, the only comparativesupport not requiring utilization of a halogenated inhibitor, reveals amost striking improvement in reaction temperature and productivity.Table 11 reveals that inventive support D may be employed at a reactortemperature 25 C lower than in the case of inventive support C withlittle reduction in productivity, and at a temperature of 15 C lowerwith a substantial increase in productivity.

Having determined the new results and advantages of the invention underprimary reactor conditions, comparative data was sought at purge reactorconditions. Accordingly, feed gas comprising 1.4 percent ethylene, 6.0percent oxygen, 7.0 percent carbon dioxide, 0.001 percent ethane balanceessentially nitrogen, was passed at a flow rate of 37.5 lbs/hr/tube anda pressure of 315 psia through beds of catalysts prepared with supportpellets AB and D. No halogenated inhibitor was used. The results interms of ethylene conversion and ethylene oxide productivity are givenfor different reactor temperatures in Table III. The term conversion" isemployed to indicate the percentage of ethylene which is converted,i.e., without regard to the products resulting from such conversion. Asa purge reaction generally represents the last opportunity to wrestproduct from the raw materials, a high conversion value is of utmostimportance.

Recourse to Table 111 reveals most dramatically that the catalystprepared with support pellets D and illustrative of the inventionpermits the obtention of highest conversion and productivity values andat lowest reactor temperatures. It is apparent that significant economicadvantages will attend practice of the invention since the data of Table111 attests to a minimum ethylene recycle to the purge reactor as wellas a maximum life of the catalyst employed therin.

While the invention has been described and disclosed in connection withvarious specific embodiments thereof, it is understood that nolimitations or restrictions are intended thereby and that saidembodiments are merely indicative of the best mode of practicing theinvention, the embodiments thereof in which an exclusive property orprivilege is claimed being defined as follows in the appended claims.

What is claimed is:

1. A catalyst for use in the controlled partial oxidation of ethylene toethylene oxide, said catalyst comprising a catalyst support impregnatedwith silver, said catalyst support consisting essentially of inert,porous particles having a minimum apparent porosity of about 30 percentand wherein at least percent of the pores have diameters in the range ofl to 30 microns, the average of said diameters being in the range of 4to 10 microns.

2. A catalyst according to claim 1 wherein said particles comprisesilicaOalumina.

3. A catalyst according to claim 2 wherein said silica is in the rangeof 1 to 6 percent by weight of said support particles.

4. A catalyst according to claim 2 wherein said particles comprise 0.1to 0.4 percent by weight baryta.

2. A catalyst according to claim 1 wherein said particles comprisesilica-alumina.
 3. A catalyst according to claim 2 wherein said silicais in the range of 1 to 6 percent by weight of said support particles.4. A catalyst according to claim 2 wherein said particles comprise 0.1to 0.4 percent by weight baryta.